Spectroscopic and potentiometric study of aromatic alpha-hydroxy azo compounds and their copper(II) complexes

Three aromatic alpha-hydroxy azo dyes, 4-(2-hydroxy-1-phenylazo)benzenesulfonate, 4-(2-hydroxy-1-naphthylazo)benzenesulfonate and 4-(9-hydroxy-10-phenanthrylazo)benzenesulfonate and their copper(II) complexes have been studied in aqueous solution. The existence of both ground- and excited-state proton transfer leading to equilibria between azo-enolic and keto-hydrazonic tautomers in the free ligands has been investigated using C-13 NMR, electronic absorption and fluoresence emission spectroscopies, together with semiempirical MO/CI calculations. The relative stabilities of the tautomers is explained on the basis of the possibility of aromaticity transfer. The formation constants of the Cu-II complexes have been evaluated using a combination of potentiometric and spectrophotometric titration data. The relative stabilities of the complexes formed with the studied ligands is also discussed.